The .mtx files are multiple (tga?) image files compiled into 1 file as the bat files say. Given the license agreement and the unobscured nature of the files, someone should email the collective and ask if they could dole out some file specs or even the mtx.exe that they used to compile the 'MULTITEX' files. Its highly unlikely that you'd get a helpful response but its worth eliminating the simple before working out the file specs.

Its a custom format, ive no doubt that it can be converted back to the individual images however for that someone needs to work out the .mtx file-format. Having the original mtx tool as I suggested before would of course make this much easier since you could compile a known file into an mtx and see how it got stored. Getting mtx.exe is, as I said extremely unlikely and someone would need to ask the collective for it anyway.